Analysis of a Mars-Stationary Orbiting Microwave Power Transmission System

NASA Technical Memorandum 101344

Kenwyn J. Long Lewis Research Center Cleveland, Ohio

July 1990

ANALYSIS OF A MARS-STATIONARY ORBITING MICROWAVE POWER TRANSMISSION SYSTEM

Kenwyn O. Long National Aeronautics and Space Admlnistration Lewis Research Center Cleveland, Ohlo 44135

SUMMARY

An efficient Mars-stationary orbiting microwave power transmission system can fulfill planetary exploration power requlrements. Both nuclear power gen- eratlon and photovoltaic energy conversion have been proposed for the orbiting power source. Thls power is to be converted to RF energy and transmitted to the surface of the planet, where it is then to be converted to dc power.

An analysis was performed for example systems at 2.45 GHz, where rectenna technology Is currently well developed, and at several higher frequencies in the 2.45- to 300-GHz range, where small antenna requirements make the system more viable. cO Thls analysis demonstrates that while component efficiencies are high at ! I,l 2.45 GHz, antenna dlmensions required to deliver a desired power level to the planetary surface are unachievably large at that frequency. Conversely, as the operating frequency is raised, component efficiencles fall, requirlng an increase in source power to achieve the desired rectified power level at the surface. Efficiencies of free electron lasers operating in the 30- to 200-GHz range are currently low enough to offset any advantage derived from the highly directional laser output beam.

State-of-the-art power transmitters at 20 to 30 GHz have moderate component efflciencles and provlde fairly high output power levels. In this frequency range, the antenna dimension requirement is approximately one-tenth that of a 2.45-GHz system. These factors, along with current rectenna development efforts at 20/30 GHz, make the 20- to 30-GHz range most desirable for Inltial microwave power transmlssion system development.

Both parabolic and phased-array transmitting antennas were investigated. The number of phased-array elements required to prevent grating lobe interfer- ence at these high frequencies may present significant phase control problems unless an appropriately dimensioned phased-array-fed reflect:or system can be designed.

Development of rectenna technology at 20 GHz and above is of prime impor- tance in achieving realistic transmitting antenna dimensions. Large, high- gain transmitting antennas are necessary to p_'ovide a power flux density high enough to meet the threshold power density criteria of the _ectenna dipole elements. INTRODUCTION

To determine the feaslb111ty of provtdlng efficient RF power transmlsslon from a Mars-statlonary (areosynchronous) orbit to the surface of the planet, an assessmentwas made focusing on RF propagatlon In the 2.45- to 300-GHz range. In order to provlde a rectlfled load power of I0 to 100 kW at the surface, generated power of approximately lO MW to 1.5 GW was considered.

The proposed orblting system conflguration, described In the first sec- tlon, provides for power generat|on by either photovo]talc array or nuclear reactor, the conversion of the dc output to RF, and the subsequent propagation of RF energy from the orbltlng array to the Martian surface. On the planet, a rectenna array wl]l convert RF to dc power to be distributed for planetary power needs.

The state-of-the-art of component technologies applicable to the orbiting microwave power transmission system Is presented in the second section. Contlnuous-wave power transmitters within the desired frequency range were assessed with regard to operating efficiency, peak power output, and magnetic field requirements.

The thlrd section derives the total efficiency of the energy conversion chain from dc to RF In orbit through RF to dc on the planetary surface for a 2.45-GHz system, which Is the sole frequency for which dlpole-element rectenna technology has been developed to date. Because it would not be possible to accurately project rectenna efficiencies for higher frequencies, the efficiency of the energy conversion chain is calculated only up through the Incident RF power at the surface of the rectenna. Tradeoffs between component efficiency and transmitting antenna requirements were considered for each of several representative frequencies within the 2.45- to 300-GHz frequency range. Receiving antenna criteria were determined from desired received power levels and rectenna element power density threshold at 2.45 GHz.

Recommendations are presented for research into developlng technologies which may afford enhanced vlabIllty of the proposed microwave power transmlsslon system.

MARS-STATIONARY ORBITING MICROWAVE POWERTRANSMISSION SYSTEM

System Design

Figure 1 Illustrates the proposed design of the Mars-stationary orbltlng microwave power transmission system. A nuclear reactor or photovoltaic array wlI] be placed in orbit about 17 000 km above the surface of the planet. A power distrlbution array will then transfer the dc power from the source to a power transmitting device, or to an array of devices, for conversion from dc to RF energy. A transmitting antenna array will then propagate the RF energy through the Martian atmosphere down to an array of rectennas on the surface of the planet. Thls rectenna array will convert the received energy from RF to de, where it will be distributed as dc power for planetary use. For simplification, in this analysls the receiving antenna array Is taken to be centered at the subsatelllte point (i.e., at a point on the Martian equator). Transmission to surface points other than this would Increase the transmission path length and atmospheric attenuatlon.

Radlo Frequency Energy Attenuation In Martian Atmosphere

The physical and chemlcal properties of the Martian atmosphere were assessed to determine extinctlon effects on RF transmissions from Mars-statlonary orbit down to the surface of the planet. These properties include atmospheric composition, absorption characteristics, and scattering effects due to windborne dust particles.

The composition of the atmosphere is approximately 95 percent carbon dioxide (C02), 2.7 percent nitrogen (N), 1.6 percent argon (Ar), and trace gases of oxygen (02), carbon monoxide (CO), neon (Ne), krypton (Kr), xenon (Xe), and ozone (03).

Relatlve concentrations of these molecules are

[COl:[CO 2] = 10-3

[02]:[C02] = 1.3xlO -3

[03]:[C02] : 2xi0-6

Atmospheric pressure on Mars ranges from 5 to 8 mbar as compared with the l.Ol3 bar Earth value.

Water vapor appears to be a highly variable component of the Martian atmosphere. The north polar cap is believed to be water ice, which partlally sublimates in the summer to release water vapor into the atmosphere. Although water vapor has been detected in the Martian atmosphere, the concentrations of other species of hydrogen and oxygen (H, H2, OH, and H202) are apparently below the detection threshold of the Vlklng lander instruments. Since the concentration of water vapor varies widely both seasonally and geograph|cally, it is not possible at thls time to assess the degree of attenuation due to that molecule. However, since the overall concentration appears to be quite low, attenuation at the two H20 absorption lines within this band (22 and 180 GHz) should be minimal.

Although molecular oxygen, 02 , has absorption lines extending from 53.5 to 65.2 GHz and at I18 GHz, this molecule exists only in trace quantities. Atten- uation at these frequencies should therefore be negligible.

The majority of the absorption lines for CO 2, the predominant atmospheric constituent, fall within the infrared region between 20 and 150 THz (15 and 2 _m). This is well above the l- to 300-GHz (3xlO -l to IxlO -3 m) range of investigation. It should be noted, however, that complete absorption data for the Martian atmosphere are not currently available because the Viking lander data have not been completely analyzed to date.

Estimates of Martian windborne dust particles range in diameter from 0.I to 30 pm (ref. I). Scattering of transmissions in the I- to 300-GHz range is not expected to be of concern because the wavelengths correspondlng to these frequencies (3xi0 -I to IxlO -3 m) are much larger than the dust partlcle diameters. However, heavy accumulatlon of dust on surface-based rectenna elements Is expected to cause scattering of Incldent RF radlatlon.

Orbiting Power Source

To meet the power requirements of Mars planetary exploration by the proposed microwave power transmission system, it has been estimated that the orbiting power source must be able to gener3te between 1MW and 1GW of dc power.

Nuclear generators and photovo]talc arrays have been proposed as power sources. Nhile each has its respective merits and disadvantages, it is not within the scope of this inltlal study to assess these power sources for use In the power transmlsslon system.

TECHNOLOGY APPLICABLE TO ORBITING MICROWAVE POWER TRANSMISSION SYSTEM

Direct Current to Radio Freauency Conversion and Transmission

For efficient transmission to the surface of Mars, the dc power generated at the orbiting source must be converted to RF. This dc to RF conversion and transmisslon may be achieved by any one of several devices which, according to the associated phase velocity, are categorized as slow-wave or fast-wave devices. Selection of the appropriate power transmitter is a function of operating frequency, gain requirements, noise limitations, and efficiency.

Slow-wave devices I (phase velocity < c). - Slow-wave devices include magnetrons, amplitrons, klystrons, and travellng-wave tubes.

Magnetrons can provide up to I kN of peak power at 2.45 GHz with an efficlency of 0.7 percent. They are relat_ely low in weight, but have a low degree of frequency stability.

Although amplitrons offer high efficiency (~80 percent) and need no active cooling, they have a low-gain, low-power output with a high degree of noise. These devlces are constrained to frequencies less than 10 GHz.

Klystrons, by comparison, offer high gain, a high degree of phase control, and low noise. If depressed coliectors are incorporated into the klystron tube design, efficiencies of 80 to 85 percent can be obtained. However, for high- power tubes (peak power > 50 kN for continuous-wave (cw) operation), an active

IKosmahl, H.G.: Microwave Power Tubes for Martian Power Station. Hand- out pr-esented at the Mars Power Transmission Seminar, NASA Lewis Research Cen- ter, Cleveland, OH, August 5, 1987.

4 cooling system Is required and the power per unlt must not be too small. The frequency range of klystrons is approxlmately 1 to 50 GHz.

Travellng-wave tubes (TWT's), by themselves, have been deemed relatlvely Inefflcient for this appllcatlon, but may enhance system efflclency when Inte- grated into the design of other transmitting devices, such as gyro-travellng wave tubes.

Fast-wave devices (phase velocity Z c). - Power transmitting devices which have a phase velocity equal to, or greater than, the speed of llght are termed fast-wave devices. These currently include gyrotrons and free electron lasers (FEL's).

Gyrotrons' The gyrotron is the most likely candidate device for power transfer in the millimeter wave range because of its high efficlency and abil- ity to operate In the cw mode. Gyrotrons, however, have high magnetlc field requirements.

Since the cyclotron frequency

(I) oc= e- where

2_f

magnetic fleld, Wb/m2 (See note. 2)

charge-to-mass ratio for the electron (1.758xi011C/kg for m = mo, where mo : rest mass) the required magnetic field for a given operatlng frequency _ can be determined by

(2)

Rest mass is used here as an approximatlon. In general, the velocity v of electrons withln a gyrotron becomes a significant fraction of the speed of light, so that the relativistic mass applies.

-1/2 (3) mre I : m° 1 -

2Note that 1Wb/m 2 = I V-sec/m 2 = 1 T : 104 G. From equation (2), a table of magnetic fleld requirements as a functlon of fundamental cyclotron resonance frequency is obtained (table I). A graph of thls function is presented in flgure 2.

These hlgh magnetic field levels requlre that for high frequencies (usually defined as belng above 60 GHz) the gyrotron operate either with super- conductlng magnets, or at harmonics of the cyclotron resonance frequency _c. Operation at a harmonlc frequency reduces the requlred magnetic field by a factor approximately equal to the harmonic number, thereby overcoming the need for superconducting magnets. Beyond the second harmonic, however, efficiency rapldly degrades.

A 500-GHz experlmental gyrotron has been developed (Heinen, V.; and Delayen, J.: Applications of High Tc Superconductors to Electromagnetic Power Transmlsslon. NASA Internal Memo, Mar. 1988.), but operation is currently limited to the pulsed mode, where peak output powers are generally higher than achievable by cw operatlon at the same frequency. Continuous-wave gyrotrons have achieved 200 kW peak power at 140 GHz, with an efficiency of approximately 28 percent. The use of hlgh-crltical-temperature superconducting materials in solenolds (Heinen, V.; and Mercereau, J.: HTS and Microwave Power Transmlsslon. NASA Internal Memo, Nov. 1987) may be deslrable to produce the large magnetic fields required above 140 GHz, without necessitating operatlon beyond the second or third harmonlc.

Free electron lasers: State-of-the-art free electron lasers (FEL's) have operatlng frequencies ranging from 30 GHz in the mlcrowave range up to the visible region of the spectrum. Although 30-GHz-pulsed FEL's have achieved peak powers of I GW (ref. 2), the pulsed nature of their output could not provide the steady supply of power necessary for the proposed power transmission configuration. Peak output power levels decline as the FEL operating frequency Increases. Current pulsed FEL's have efficiencies of less than lO percent. Continuous-wave FEL operation may be achievable; output power for the cw devices would be substantially lower than for pulsed FEL's of identical operating frequency.

Two constraints exist in the generation of hlgh-energy electrons for FEL operation: a linear accelerator is necessary for the production of the electron beam, and the output wavelength of the FEL decreases as the square of the beam energy requirement increases.

Solid state devices for power transmission. - Solid state devices have been considered for power transmission appllcations; however, the high- frequency limit for solid state devices ranges from 150 GHz for solid state Gunn oscillators to about 300 GHz for IMPATT 3 sources. Both the power output and the efficiency of electron tube devices far exceed that of current solid state technology, as does the upper frequency limit (ref. 3).

31nteraction of impact ionization avalanche and transit time of charge carriers. Receiving Antenna Array

A receiving antenna array (rectenna) located on the surface of the planet will recelve microwave radlatlon from the Mars-statlonary orbiting power source. Thls array wlll convert the incident RF power Into dc electrical power to meet planetary exploration needs.

When Interception of the transmitted power is greater than 80 percent, a circular receiving array Is more efficient. For interception efficlencles below that figure (as associated wlth microwave power transmission over long dlstances) a rectangular receiving array would be the more llkely candidate.

Dipole-element rectenna development. - Dipole-element rectenna technology is currently well developed at 2.45 GHz, where an overall efficiency of approximately 80 percent has been achieved (ref. 4). At thls frequency, however, array dimensions required for efficient energy interception are large, as demonstrated in the section Determination of Free-Space Interception Effi- ciency, and lead to a high degree of pointlng error.

To reduce the required array dimensions and increase interceptlon efficiency, it may prove desirable for the orbiting power transmitter to operate at a hlgher frequency. Investigation has been made Into the design of a 20-GHz rectenna, frequency-scaled from the current 2.45-GHz rectenna design. However, the scaling factor Is qulte large (o = f/fo = 8.16). Since the number of elements per square meter Is dependent upon o2, the resulting 20-GHz design would require approximately 13 300 elements per square meter, as opposed to the 200 per square meter required at 2.45 GHz. If this approach were extended to 300 GHz (o = 122.45), then 200 (02), or approximately 3 000 000 e?ements, would be required per square meter. Other approaches are being considered, includlng a completely monolithic design in which both the diodes and the circuitry would be built on a gallium arsenide (GaAs) or silicon (Si) substrate.

Wlth increased operating frequency, associated dlpole-element spacing decreases. This glves rise to undesirable inductive and capacitative effects whlch degrade efflclency. Scattering of incldent radlatlon from the rectenna edges may present a problem at shorter wavelengths.

As discussed further in the section Power Flux Density at Receiving Array, a minimum power density must be incident upon the dipole elements in order to effect rectificatlon.

ANALYSIS OF POWER TRANSMISSION SYSTEM EFFICIENCY

Several representative frequencies in the 2.45- to 300-GHz range were chosen for assessing overall efficiency of power transfer from the orbiting source to the output of rectified power at the receiving antenna array on the surface.

Calculations were made of required source power, incident power flux density at the surface, and antenna gain and dimension requirements. By using these parameters along with efficiencies of state-of-the-art and projected system components, interception efficiency n(_) was calculated using both manual methods and an analysis program developed by Grady H. Stevens of the NASA Lewls Research Center for use on the IBM-PC. Thls program was sllghtly modlfled by the author for the range of load powers In thls study. Data from thls study are presented In the section Analysis of System Performance.

Radio Frequency Transmitting and Receiving Components

Transmlttlng antenna. - Transmission of RF energy from the output of the orbltlng power generator to the receiving array on the surface of Mars Is to be effected by elther a parabolic reflector or a phased-array-fed reflector system. An aperture fleld dlstrlbutlon of f(r) = (l - r2)_, as produced by a feed horn array, is assumed for either type of antenna and is associated with an aperture efficiency (or gain factor) of 56 percent.

Assessment of phased array and parabol|c reflector transmitting antenna. - Phased-array antennas offer the advantages of electronlc beam steering and beam shaping but entail high cost due to the phase shifters required for each element in the array (a very large number at mlcrowave frequencles). In addltion, spurious radlatlon arises between the feed networks, and undesirable coupling occurs between the antenna elements, thereby degrading efflciency. In general, the galn achleved by a phased array is less than that of a parabolic reflector antenna of the same dimension. The phase locking necessary for phased arrays of gyrotrons and free electron laser sources has not been achieved to date.

The extreme number of antenna elements necessary to avoid grating lobe contribution to the maln beam of each element make large, stand-alone planar phased arrays impractical In the 2.45- to 300-GHz range. Phased-array-fed reflector systems have been suggested as a means of reducing element requirements; the dimensions of the phased-array feed are on the order of those of the subreflector.

To avoid gratlng lobes, which would reduce the main beam power and conse- quently the antenna gain, no gratlng lobe may exist within a circle of radius r = l + slne m, where e m is the maximum scan angle. From the Mars-stationary orbit, the maximum scanning angle from the equator toward one of the poles is approximately 9°. Under this condition, r : 1.156. If a square lattice is assumed, then

X X (4) dx - aJy - 1.156 and the required element spacing becomes

d = 0.865 _ (5)

A 30-GHz (X = 0.01 m) phased-array-fed reflector system with a subreflector diameter of 50 X (D = 0.50 m) would require approximately 3340 elements under this grating ]obe avoidance constraint. If coupled with a 56-percent efficient 4.25-m parabolic dish, a directive gain on the order of 60 aB could be achieved. By contrast, a stand-alone planar phased array would require approximately 1.4×lO 5 elements to meet this galn level. Calculatlon of Required Source Power

The source power required to provide a specified recelved (load) dc power can be ascertained by

PL nsystem - Psource (6)

The system efficiency is

(7) nsystem : (n(_))(nDC_DC)(nDC÷RF)(nRF÷DC)

where

n(_) Interceptlon efficlency

nDC+DC efflclency of transmission from power source to power transmitter nDC_RF power transmitter conversion efficiency nRF_DC rectenna efficiency

Power Flux Denslty at Receiving Array

A dlpole-element rectenna array has a characterlstlc threshold level of incident power per unit area (power flux density) below which the elements can- not achieve rectification of the received RF energy. Element efficiencies have been measured for 2.45-GHz arrays (ref. 5) as illustrated in figure 3.

Incldent power flux denslty (W/m 2) can be determined from the transmitted power and transmitting antenna gain"

PT g PFD = (8) 4_L2 where

PT transmitter output power, W

transmlttlng antenna gain, numerlcal

antenna separation dlstance, m, 17xlO 6 m for Mars-synchronous orbit

The assumption Is made in equatlons (9) to (11) that the rectenna array is located in the far field of the transmitting antenna, so that L >-- (9) - X

From figure 3, based upon data derived In a study of a proposed space-to-Earth power transmission system (ref. 6), a rectenna element efficiency of 78 percent requIres a PFD of approximately lO mW/cm 2 (lO0 W/m2). To determine the transmitting antenna diameter DT required to meet thls threshold PFD, the transmltter output power PT is used In the relationship for the area A T of the transmitter

(X2L2)(PFD) (I0) AT = nTP T where nT efficiency (gain factor) of transmitting antenna (assuming a clrcu]ar antenna), so that

112 = 2LXrPFDlI/2 (II) L_PTnTJ

Dielectric breakdown of the air at Earth's atmospherlc pressure occurs at power densltles of about lO 5 to lO 6 W/cm 2. In the Martian atmosphere, which averages 0.004 that of the Earth the breakdown power density would be on the order of 5xlO _ W/cm 2 (5xi06 W/m2i (ref. 6). This Is well above the maximum PFD levels created by microwave transmlsslon at or below 1.5 GW source output power at an antenna separation distance of 17 000 km, as In thls study.

Minimum Receiving Array Dimenslon

Planetary appllcatlons of the proposed microwave power transmission system require that, as a minimum, a specifled level of dc power be available at the output of the rectenna. This is referred to as the load power. The level of RF power Incldent upon the rectenna array must therefore be greater than the required dc load power by a factor of the reciprocal of the rectenna conversion efficiency. The received power requirement Is then

PL (12) PR - nRF.D C where

PR RF power input to the rectenna, N

PL dc load power at output of rectenna, W nRF.DC rectenna conversion efficiency

I0 A known power flux density, along wlth the recelved power requlrement, can be used to determine the recelvlng antenna array dlmenslon requirement

PR (13) A R - PFD

where

A R minimum array area, m2

PR power intercepted at the rece_vlng array, W

PFD power flux density at the surface of the recelvlng array, W/m 2

Since the rectenna array is assumed to be square for this application, the diameter or edge dimension of the array is

DR(m) : _ (14)

Determination of Free-Space Interception Efficiency

Assuming a clrcular transmitting antenna and a rectangular receiving array, the free-space interception efflclency n(_) (the fractlon of transmitted energy intercepted by the receiving antenna), can be obtained by calculat- Ing the free-space transmission interceptlon factor (refs. 7 and 8).

(_5)

where

AT area of transmitting antenna

AR area of receiving antenna array

L antenna separation distance = 17xlO 6 m for a Mars-stationary system

The free-space Interception efficiency is (refs. 7 and 8)

(_2) (16) n(m) : I - e which, for small _, represents the optimaI power coupling efficiency of a Gaussian main beam focused at the receiver.

A large value of _, obtained either by means of an extremely large antenna dimension or by a comparatively short antenna separation distance, would indi- cate that most of the transmitted energy is collected, and therefore q(_), the Interceptlon efficiency, would be on the order of unity. Small _ values cor- respond to the receiving antenna's intercepting only a small fraction of the main beam.

II Several example microwave power transmlsslon systems were modeled by using the relatlonshlps in equatlons (6) through (16) along with rectenna efflclency values from flgure 3. Results For 2.45-GHz systems are presented In table II. A requlrement of 10 kW dc load power on the Martlan surface, with lO0 W/m 2 power flux density incident upon the rectenna array, would require approximately 1.429 GW of dc source power coupled to a transmlttlng antenna 693 m in dlameter. This system necessitates a rectenna array area of 127 m 2 (11.3 m on each side). The effects of varylng load power and incident PFD are also given In table II.

As shown in table II, a greater fraction of incident RF energy can be intercepted when the criterion for incident PFD is dropped from I00 W/m2 to ]0 W/m2. Thls increase In q(_) outweighs the corresponding reduction in rec- tenr_a element efficiency resulting from operation at the lower PFD value. In thls analysis, dlode-element efficiency was used to approximate the efficiency of rectification. For a fixed PFD level, an increased load power requirement mandates enlarging the receiving array dimension, thereby Increasing the interception efficiency.

From thls analysis, the set of parameters yielding the highest interceptlon efflclency (PL : 100 KW, PFD = 10 W/m 2) was used as a guideline for com- puting comparative interception efflclenc1es and required source power for systems operating wlthin the 2.45- to 300-GHz frequency range. These data, presented in table Ill, demonstrate that whlle (for a fixed transmitting antenna gain level) requlrements drop with rlslng frequency, the transmitted power efflclency nTp

(17) nTp = n(_) x nDC_D c x nDC_R F

decreases as a result of decreasing power transmitter device efficiency, nDC÷R F, since n(_) and nDC÷DC are constant. Thls device efficiency, however, is subject to improvement with the development of device technology.

Transmitted power efficiency, the fraction of dc source power which is intercepted as RF power by the rectenna, was calculated. Interception efficiency n(_) Is a function of the Interception factor _, as defined by equa- tlon (15), and is thus dependent upon the area of the receiving rectenna array.

The transmltting antenna diameters used in table III were based on the assumption of an approximate lO0-dB antenna gain as eventually achievable for frequencies greater than or equal to 20 GHz. Below 20 GHz, such high-gain antenna dimensions may prove impracticable. A lO0-dB gain transmitting antenna for a 2.45-GHz system, for example, requires a diameter of almost 3 km; therefore, an 85-dB gain antenna with a diameter of 693 m, was assumed. An analo- gous 30-GHz system could be achieved with a lO0-dB gain antenna with a diameter of approximately 240 m, less than one-tenth the requirement for a 2.45-GHz transmitting antenna of the same gain. Additionally, power transmitters in the 30-GHz frequency range have demonstrated fairly high peak output power levels (on the order of 300 kN for cw operatlon). Their currently moderate component efficiencies may be raised with development. These factors, along with ongoing rectenna research at 20/30 GHz, suggest that 30 GHz may be desirable for ini- tial microwave power transmlsslon system development. Component technology at higher frequencies would require conslderably more development to bring effi- clencies up to those currently avallable at 2.45 GHz.

12 These system performance analyses demonstrate that large, hlgh-galn transmitting antennas would be necessary to provide power flux density levels sufficient for rectification of Incldent RF radlatlon at the Mars-statlonary orbltlng system range. Although source power requirements are extremely hlgh and component efflclencles range from 30 to 90 percent for frequencies below 140 GHz, the antenna separation dlstance severely impacts the transmitted power efficiency nTp, and in turn the interception efficiency n(_).

CONCLUSIONS AND RECOMMENDATIONS

The range-spreadlng inherent in a Mars-statlonary orblting system mandates the use of e|ther large, very-hlgh-galn transmitting antennas or transmitter output power levels In the multi-megawatt to glgawatt range to provide power flux density levels above the threshold necessary for the rectlfication of recelved power.

For state-of-the-art power transmitting devices, efficiency declines with rising frequency. Experimental gyrotrons operating at 140 GHz have peak efficiencles of less than 30 percent. Beyond this frequency, magnetic fleld requirements currently make operation at higher harmonics necessary, thereby degradlng efficiency. Use of hlgh-crltIcal-temperature superconducting materials In gyrotron deslgn may allow hlgher magnetic fields to be achleved, obvlatlng the need for operation above the fundamental frequency. In an array of power transmitters, phase and amplitude control are necessary to achieve a unlform dlstrlbutlon of output power.

To enhance component efficlency, it may be desirable to integrate depressed collectors into the design of power transmitters. However, the degree of such efficiency enhancement may decline with Increasing frequency.

Free electron lasers capable of producing microwave emissions are currently limited to pulsed, low-eff|clency devices. Linear accelerators are necessary to produce the required electron beam.

The development of rectennas for frequencies above 2.45 GHz will allow for a reduced transmitting antenna requlrement in the proposed system. The operating frequency must be low enough, however, to avoid undesirable coupling effects whlch would arlse if the rectenna elements were very closely spaced. Efforts are ongoing in the development of 20/30-GHz rectenna technoTogy. State-of-the-art power transmitters provide fairly high output power levels in the 20- to 30-GHz range with moderate component efficiencies, and have antenna dimension requirements on the order of one-tenth that of a comparable 2.45-GHz system. These factors make the 20- to 30-GHz range appear most desirable for inltial development of a microwave power transmission system.

Developments in solid state device technology may provide enhanced rectenna efficiency. For optimum element efficiency, each dipole and associated Schottky detector must have a high degree of impedance matching. Mono- lithic circuitry has been suggested as a means of enhancing impedance matching. Diode sensitivity can be improved with a gallium arsenide heterostructure, thus leading to increased rectification efficiency.

13 ACKNOWLEDGMENT

The author Is grateful for the guldance provlded by Dr. Afroz Zaman, of the Antenna and RF Systems Technology Branch of the Space Electronics Divlslon, In assesslng appllcatlons of phased-array antenna technology to the proposed orbltlng power transmlsslon system.

14 APPENDIX - SYMBOLS

Aem maxlmum effective area

AR area of receiving array

AT area of transmlttlng antenna

B magnetic fleld

C speed of light

DR edge dlmenslon of recelvlng array

DT transmitting antenna diameter d phased-array element spacing f frequency

reference frequency for scaling

G gain of transmitting antenna, dB g galn of transmitting antenna, numerlcal

L antenna separation distance m mass mo rest mass mre] relativistlc mass

N number of elements

PFD power flux density, W/m2

PL load power

PR received power

Psource source power

PT transmitter output power

[q/m] e_ electron charge-to-mass ratio

R radius of circular transmitting antenna

radius

15 nDC_DC efficiency of transmlss|on from power source to power transml tter nDC÷RF power transmltter efflclency nRF÷DC rectenna effIclency nsystem overall system efflclency nT transmitting antenna efficiency nTp transmitted power efficlency n(_) Interceptlon efficiency

Qm maximum scan angle of phased array o frequency scallng factor

free-space transmission interception factor

_C cyclotron frequency

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6. Scharfman, W.E.; Taylor, W.C.; and Mor|ta, T.: Breakdown Limltatlons on the Transmlsslon of Microwave Power Through the Atmosphere. IEEE Trans. Antennas Propagat., vol. AP-12, no. II, Nov. 1964, pp. 709-717.

7. Goubau, G.: Microwave Power Transmission from an Orbiting Solar Power Station. J. Microwave Power, vol. 5, no. 4, Dec. 1970, pp. 223-23].

8. Manning, R.M.: Optimizing the Antenna System of a Microwave Space Power Station: Implications for the Selection of Operating Power, Frequency, and Antenna Size. NASA TM-]OOI84, 1987.

I7 TABLE I. - GYROTRON MAGNETIC FIELD REQUIREMENT

Frequency, Magnetic field, f, B, GHz Wblm 2 (or T)

1 0.03 2.45 .088 30 1.072 100 3.57 200 7.15 300 10.72

TABLE II. - INTERCEPTION EFFICIENCY OF 2.45 GHz SYSTEMS

[Transmitting antenna: area, 3.77x105 m2; diam, 692.84 m; gain, 85 dB; efficiency, 56 percent, qDC+DC and qDC_RF, 0.9.]

Incide'nt Receiving Load Interception Interception Rectenna System Required PFD. array_area, power, factor, efficiency, efficiency, efficiency, source power, W/m Z m _ kW MW n(_) qRF_DC qsystem

10 1 786 10 1.25x10 -2 1.55xlO-_ 0.56 143 I0 17 860 100 3.94xI0-_ 1.55x10-_ .56 _.OxlOOx]O -4-5 143 100 127.06 10 3.3x10-_ 1.1xI0-5 .787 710x10 -6 1429 I00 1 270.6 I00 1.05x10 -Z l.lxlO -4 • 787 7.0xlO -5 1429

TABLE III.- SYSTEM PERFORMANCE ANALYSIS

[Load power, PL _ 100 kW; incident RF PFD : I0 W/m2; receiving array area, AR = 17 860 me; qOC,OC = 0.9; qRF_OC : 0.787 assumed for all frequencies for load power approximatTon; antenna efficiency = 56 percent; transmitting antenna gain_: 85 dB for <20 GHz (1QO dB for > 20 GHz); interception factor, T = 3.94x10 -Z fqr <20 GHz (1.66x10 -_ fqr _ GMz); interception efficiency, q(_) = 1.55x10 -° for <20 GHz (2.72x10 -L for _ 20 GHz).]

Frequency, Transmitting Transmitting Power Transmitted Required GHz antenna_area, antenna transmitter power source AT(m Z) diam, efficiency, efficiency, power, MW DT(m) qDC_RF qTp a

2.45 3.77x10 _ O. 90 ).26xI0-) 100.85 20 1.OOxlO _ .93xl0_57x10 Z .65 1.5gxlO-_ 7.99 30 4.45xi0 z 2138xI0_ .53 1.30xlO-_ 9.77 60 1.11xiO! .50 1.22xI0-_ 10.42 100 _.19xlO.14xlO Zl •40 .glxlO-_ 12.97 4.00xI0_ 140 2.04xi0 5.10xlO l .28 .69xI0-_ 18.42 200 3 57xi01 .25 .61x10-_ 20.83 300 .45x10.OOx10}Z 2 38xi01 .20 .49x10 -2 25.93

anT P : q(_) x qDC_DC x qDC_RF"

18 I)C POW[R SOIJRCF NI.ICI.[AR OR PIIOIOVOI IAIC

SIRJI_urIONN,I [ CONViRSION J ORBIIING(17POWIRo00KM)SIAIION

_F POWER _ISIRIBU_ lo_

IRANSM[I ] IN(; ANI[NNA

IIIGII POWI R MICROWAVE R{ AM

I0 APPI ICAI IONS

I I(;IlR[ 1. SCFNARIO Ol MARS STATIONARY OREIIIN(; MICROWAV[ I'OWIR IRANSMISF;ION SY*,;IIM.

12-- 120 --

10-- 1oo-- o ,.,.: / • 8 -- '_ 80-- =

6--

L_J lie

2 m

2°L/_ L I I I l OL I 0 50 l O0 150 200 250 300 150 I:R[OU[NLY, GIIz

I I(;UR{ 2. GYROIR()N MA(,NI IIC t If If) REOUIREMLNI AS [UN( lION Ol FRIQUINCY

19 100

• GO _

8o 40

0 i * I II,l,i I I I *l*lll I I I ,IJlll .2 2 20 200 POWERDENSITY, m//cN 2 FIGURE 3. - RECTENNAELEMENTEFFICIENCY AS FUNCTION OF LOCAL POWER DENSITY (REF, G).

2O Report Documentation Page Nalional Aetonat/tics and Space Administration

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. NASA TM-101344

4. Title and Subtitle 5. Report Date

Analysis of a Mars-Stationary Orbiting Microwave Power July 1990 Transmission System 6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.

Kenwyn J. Long E-4367

10. Work Unit No.

643-10-01

Performing Organization Name and Address 11. Contract or Grant No. National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio 44135-3191 13. Type of Report and Period Covered

Technical Memorandum 12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration 14. Sponsoring Agency Code Washington, D.C. 20546-0001

15. Supplementary Notes

16. Abstract

To determine the feasibility of providing efficient RF power transmission from a Mars-stationary orbit to the surface of the planet, an assessment was made focusing on RF propagation in the 2.45- to 300-GHz range. The proposed orbiting system configuration provides for power generation by either photovoltaic array or nuclear reactor, the conversion of the dc output to RF, and subsequent propagation of RF energy from the orbiting array to the Martian surface. On the planet, a rectenna array will convert RF to dc power to be distributed for planetary power needs. Total efficiency of the energy conversion chain from dc to RF in orbit through RF to dc on the planetary surface was derived for several representative frequencies in the range of study. Tradeoffs between component efficiency and transmitting antenna requirements were considered for each of these frequencies. Rectenna element power density thresholds and desired received power levels were used to determine receiving antenna criteria. Recommendations are presented for research into developing technologies which may afford enhanced viability of the proposed microwave power transmission system.

17. Key Words (Suggested by Author(s)) 18. Distribution Statement

Microwave power transmission Unclassified- Unlimited Free-space power transmission Subject Category 31

19. Security Classif. (of this report) T 20. Security Classif. (of this page) ; 21. No. of pages 22. Price'

/ / Unclassified Unclassified 22 A03

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